Core magnetic shear effects on energetic ion drive to disruptive instabilities

Simulations of the onset of resistive MHD instabilities are presented where a slowing down distribution of energetic ions can either stabilize or destabilize disruptive tearing modes depending on the magnetic shear in the core. Two cases are compared, one with monotonic shear throughout the profile and one with reversed shear in the core. The monotonic case has a minimum q of 1.1 while the reversed case has a minimum of 1.3. Outside of the reversal the profiles and equilibrium structure are nearly identical between the two cases. The drive from energetic ions is stabilizing in monotonic shear and destabilizing in reversed shear, consistent with previous work. In the reversed shear case without energetic ions a 3/2 mode is unstable, while with ions both a 2/1 and 3/2 mode are driven. The beta limits as a function of rotation with a resistive wall are modified by the energetic ions in a reduced model depending on the magnetic shear. Nonlinear simulations with energetic ions are also explored, and analyzed in terms of the linear results and a reduced model of the energetic ion effect on the resistive mode.